THE BACK.
[Illustration: FIG. 31.--The spinal column. (Church and Peterson.)]
=The Spine.=--The trunk may be roughly divided into the back, the chest
or thorax, the abdomen, and the pelvis. By the back is denoted the
spinal column with its muscles, blood-vessels, etc., and the spinal
cord already described. The spine or vertebral column, which serves the
double purpose of holding the body erect and of protecting the cord, is
usually about two feet, two inches in length. In its course there occur
several curves, which serve to give springiness and strength and, with
the intervertebral cartilages, to mitigate the force of concussion from
blows and falls. The curve is convex forward in the cervical region,
convex backward in the dorsal, forward in the lumbar, and backward
again in the sacral region. There is most freedom of motion in the
cervical region.
As is the case with the other bones, the _vertebræ_ are specially
adapted in shape and size to the needs they are called upon to fill.
Strength and flexibility, with a minimum bulk, a channel for the cord,
and passages for the numerous nerves and blood-vessels are some of the
requirements which, in combination, they meet to an astonishing degree.
They are thirty-three in all, and are divided into groups according
to the region in which they occur: seven cervical in the neck, twelve
dorsal or thoracic, five lumbar, five sacral, and four coccygeal.
Although the vertebræ of the different groups differ more or less
in size and shape in accordance with the various demands of their
positions, they all have certain general characteristics. Each has
a body, two laminæ, two pedicles, two transverse processes, and one
spinous process. The _pedicles_ extend back from the body on either
side and support two broad plates of bone, the _laminæ_, whose juncture
at the back completes the _spinal foramen_ for the passage of the cord.
At their juncture is the _spinous process_, which can be felt beneath
the skin, while the _transverse processes_ project from the juncture of
the laminæ with the pedicles. All the processes are for the attachment
of muscles that move the spine. The _body_ is formed of cancellous bone
with a compact layer outside. Transversely it is slightly oval, while
its upper and lower surfaces are flat, except in the cervical region,
where the upper surface is concave laterally and the under convex
laterally and concave from before back. Between the bodies are disks of
fibro-cartilage, which increases motion and springiness. The _spinous
process_ or _spine_ is short in the cervical region, long and directed
downward in the dorsal region, thick and projecting almost straight
out in the lumbar region. The pedicles are notched above and below so
that when articulated the notches of two vertebræ join to form the
_intervertebral foramen_ for the outward passage of nerves and the
inward passage of blood-vessels.
The distinguishing mark of the _cervical vertebræ_ is the foramen in
each transverse process, through which the vertebral arteries run to
the skull. They are also smaller than the dorsal and lumbar vertebræ.
The _dorsal vertebræ_ are distinguished by having on the transverse
processes and on the body smooth articular surfaces called facets and
demi-facets for articulation with the ribs. The _lumbar vertebræ_ are
the largest and heaviest and have the thickest spine. By the time the
sacral region is reached, however, the vertebræ have only a rudimentary
spinous process. Moreover, in adult age the sacral bones grow together
and form one triangular bone, the _sacrum_, which has a broad base
called the _promontory of the sacrum_ and a blunt apex. It is concave
in front and convex behind and has an articulating surface for joining
the pelvic bones. In the case of the _coccyx_ also the four original
bones, all rudimentary in character and supposed to be the survival of
a tail, grow together to form one bone. Together the sacrum and coccyx
form the posterior wall of the true pelvis.
[Illustration: FIG. 32.--A type of vertebra. (Leidy.) 1, Body;
2, pedicle; 3, lamina; 4, spinal foramen; 5, spinous process; 6,
transverse process; 7, articular process.]
[Illustration: FIG. 33.--The sacrum, from before. (Drawn by D. Gunn.)]
Some of the dorsal vertebræ are peculiar in the arrangement of their
facets and demi-facets, while among the _cervical vertebræ_ are several
whose peculiarities should be more carefully noted. Thus, the first
cervical vertebra or _atlas_ supports the head and has practically no
body, the place of the body being taken by a narrow anterior arch of
bone and an opening, continuous with the spinal foramen, into which the
_odontoid process of the axis_ fits, being held in place by ligaments.
At either side on top is a facet for articulation with the occipital
bone. There is almost no spine. The second vertebra or _axis_ has
surmounting the body the odontoid process, with a facet in front for
articulation with the atlas and one behind for the transverse ligament
to move over. The seventh cervical vertebra or _vertebra prominens_
has a very long spinous process--hence name--to which is attached the
_ligamentum nuchæ_. It can be felt very distinctly on the living.
Running from the skull down through the spinal column into the sacral
vertebræ and formed by the joining of the spinal foramina of the
individual vertebræ is an opening called the _spinal canal_, which
holds the cord. The cord, however, stops practically at the first
lumbar vertebra, where it splits up into the _cauda equina_, only the
_filum terminate_ extending farther down.
Occasionally the laminæ do not form completely and the membranes of the
cord may bulge out and form a tumor, or the cord itself may come out
also. This generally occurs in the lumbar region, where it is known as
_spina bifida_. If in case of fracture of a vertebra there is paralysis
of the parts below due simply to the pressure of a fragment of bone
upon the cord, it may be completely cured by removal of the fragment.
If, however, the cord suffers injury, the paralysis will remain.
_Humpback_ or _Pott’s disease_ is caused by the tubercle bacillus,
which eats away the bodies of the vertebræ so that the column caves in
and the spinous processes are thrown out in a hump or kyphos.
[Illustration: FIG. 34.--Muscles of the right side of the head and
neck: 1, Frontalis; 2, superior auricular; 3, posterior auricular; 4,
orbicularis palpebrarum; 5, pyramidalis nasi; 6, compressor naris; 7,
levator labii superioris alæque nasi; 8, levator labii superioris;
9, zygomaticus major; 10, orbicularis oris; 11, depressor labii
inferioris; 12, depressor anguli oris; 13, anterior belly of digastric;
14, mylohyoid; 15, hyoglossus; 16, stylohyoid; 17, posterior belly of
digastric; 18, the masseter; 19, sternohyoid; 20, anterior belly of
omohyoid; 21, thyrohyoid; 22, 23, lower and middle constrictors of
pharynx; 24, sternomastoid; 25, 26, splenius; 27, levator scapulæ; 28,
anterior scalenus; 29, posterior belly of omohyoid; 30, middle and
posterior scalenus; 31, trapezius. (Dorland’s Dictionary.)]
=Muscles of the Neck.=--Before speaking of the muscles of the back
a few of those of the neck had best be taken up. They are numerous
but mostly of minor importance. Largest and most important is the
_sterno-cleido-mastoid muscle_, which has its origin on the upper part
of the sternum and the inner third of the clavicle and is inserted into
the mastoid process of the temporal bone. It passes obliquely across
the side of the neck and serves to flex the head to the side and to
draw the face in the opposite direction. When both muscles contract the
head is flexed on the neck and the neck on the chest. In _wry neck_
or _torticollis_ this muscle is constantly contracted. The _platysma
myoides_ arises from the fascia over the pectoral, deltoid, and
trapezius muscles and is inserted into the lower jaw, the angle of the
mouth, and the loose tissue in the lower part of the face. It wrinkles
the skin of the neck and depresses the lower jaw. In the cow and horse
it is so highly developed that by it the skin can be contracted all
over the body to drive off flies. The _rectus capitis anticus major_
arises from the third to the sixth cervical vertebræ and is inserted
into the occipital bone, serving to flex the head. The _scalenus
muscles_ have their origin on the lower cervical vertebræ and are
inserted into the first and second ribs, thus aiding in the elevation
of the ribs as well as in lateral flexion of the neck. The head is
held upright by the _ligamentum nuchæ_, which rises from the external
occipital protuberance and is inserted into the spinous processes of
all the cervical vertebræ except the first.
=Muscles of the Back.=--The chief back muscles are the trapezius
and the latissimus dorsi, which together cover in the back pretty
thoroughly. The _trapezius_ arises from the occipital bone, the
ligamentum nuchæ, and the spinous processes of the seventh cervical
and all the dorsal vertebræ and is inserted into the outer third of
the clavicle or collar bone and the acromion process and spine of the
scapula or shoulder blade. It is thus triangular in shape and covers in
the neck and shoulders, serving to draw the head back and to the side.
It overlaps the latissimus dorsi.
The _latissimus dorsi_ has its origin by aponeurosis from the spinous
processes of the six lower dorsal and all the lumbar and sacral
vertebræ, from the crest of the ilium or hip bone, and from the three
or four lower ribs, swings across the side, dwindling in size, and is
inserted by a small tendon into the bicipital groove of the humerus or
upper arm bone, thus covering in the part of the back not covered by
the trapezius. It draws the arm down and back, raises the lower ribs,
and draws the trunk forward, as in climbing. The flat muscles of the
back and abdomen have a tendency to flatten out into aponeuroses, such
as occurs in the origin of the latissimus dorsi.
The _levator scapulæ_, from the transverse processes of the upper
cervical vertebræ to the posterior border of the scapula, serves
to raise the angle of the scapula, and the _rhomboideus major_ and
_minor_, from the ligamentum nuchæ, the seventh cervical, and the upper
dorsal vertebræ to the root of the spine of the scapula, draw the
inferior angle back and up.
[Illustration: FIG. 35.--Muscles of the trunk from behind (left side,
superficial; right side, deep): 1, Sternomastoid; 2, splenius; 3,
trapezius; 4, latissimus dorsi; 5, infraspinatus; 6, teres minor;
7, teres major; 8, deltoid; 9, external oblique of abdomen; 10,
gluteus medius; 11, gluteus maximus; 12, levator anguliscapulæ; 13,
rhomboideus minor; 14, rhomboideus major; 15, part of longissimus
dorsi; 16, tendons of insertion of iliocostalis; 17, supraspinatus; 18,
infraspinatus; 19, teres minor; 20, teres major; 21, serratus magnus;
22, upper, and 22´, lower part of serratus posticus inferior; 23,
internal oblique; 24, gluteus medius; 25, pyriformis and superior and
inferior gemelli; 26, 26´, portions of obturator internus; 27, tendon
of obturator internus; 28, quadratus femoris. (Dorland’s Dictionary.)]
The _blood supply_ in the cervical region and about the shoulders comes
from branches of the subclavian artery, such as the suprascapular and
the transversalis colli. Lower down the supply comes from the posterior
branches of the intercostals, dorsal branches of the lumbar, and
branches of the internal iliac.
The muscles of the back are supplied by the _spinal nerves_, the spinal
accessory also going to the trapezius muscle.